Many attending a vigil Monday night marked their favorite memories of Matthew Blackford by signing his beloved Honda Civic.

Dozens gathered on the south side of Gray's Lake to honor the 21-year-old, 24 hours after he was killed while "mudding" with friends near the Des Moines River.

According to police, Blackford was driving a Jeep Cherokee which got stuck near the water.

The cause of death is under investigation, but because the exhaust pipe was underwater, police say carbon monoxide poisoning may be a factor.

"We pulled him out, put him in my Jeep after trying to give him CPR. (I) pulled my Jeep seat down and he laid on my chest. "I said, 'It's OK, man. Everthing's going to be alright. We're going to get some help," said friend Anthony Barcelo.

Blackford's cousin said the vigil's attendance is proof he will be deeply missed.

"It's just another testimony to what kind of man Matthew was. Our family is strong in faith and we got to be that way," said one family member.

Blackford leaves behind a 14-month old daughter, McKenna.

Funeral plans are pending. Family members told KCCI that they plan to set up a memorial fund in his name Tuesday at the Wells Fargo Bank on Army Post Road.